A process chamber in a furnace is heated by burners that fire into the process chamber. The burners may be provided with fuel in the form of natural gas and oxidant in the form of combustion air. Those reactants are typically provided at flow rates on the air-rich side of the stoichiometric ratio. The excess air ensures that all of the fuel has oxidant available for combustion in the process chamber. The excess air also contributes to the momentum or mass flow of combustion products to promote temperature uniformity throughout the process chamber. However, excess air can lower the efficiency of combustion in the process chamber because it carries heat out with the exhaust gases.
As the temperature in the process chamber reaches increasingly elevated levels, lesser amounts of fuel become necessary to cause further increases in temperature. The burners may then be operated at lower firing rates. The firing rate at a burner can be reduced by reducing the fuel flow rate while maintaining the air flow rate at the original level. This preserves the momentum and temperature uniformity provided by the original air flow rate, but lowers the combustion efficiency as the increasing air-to-fuel ratio includes increasing amounts of excess air. On the other hand, reducing the flow rates of both fuel and air can preserve the efficiency but has the disadvantage of lowering the momentum.